Why Do Electric Vehicles Only Have 1 Gear? – Video

3 months ago by Mark Kane 36

Engineering Explained, in partnership with the Formula E, explained the topic of a single-gear transmission, a set-up typically utilized in electric vehicles.

Why Do Electric Cars Only Have 1 Gear? (source: Engineering Explained)

There are of course examples of two and three speed transmissions, but in general EVs don’t need to change gears, as electric motors still work great with a fixed ratio.

The reasons for that are:

  • high revving, that enables to cover whole speed range using fixed gear ratio
  • high efficiency in broad revs range
  • high torque in broad revs range, especially at 0 RPM and low revs

Another reason for a single gear is the power profile of many EVs – the instant torque can really play havoc on complex transmission set-ups.

Why don’t electric cars have multiple-gear transmissions?

Why do electric cars only have 1 gear? This is the first of a five part series sponsored by Formula E, who I’ve partnered with to talk about the engineering behind electric cars. I had the opportunity to get behind the scenes at the New York City E-Prix, and was able to chat with team principals, hang out in the engineering rooms during qualifying, and even learn from this year’s champion, Lucas di Grassi.

So why do electric cars use just a single gear, rather than using traditional transmissions like you’d find paired with internal combustion engines? Electric motors can get away without numerous gears because they are high revving, remain fairly efficient across a very broad rev range, and produce a great amount of torque at low RPM.

Why Do Electric Cars Only Have 1 Gear? (source: Engineering Explained)

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36 responses to "Why Do Electric Vehicles Only Have 1 Gear? – Video"

  1. El Fantastico says:

    Great explanation and graphics! *I* even understood what he said! But don’t ask me to repeat that… 😀

  2. Mark.B says:

    Sorry, is not the circumference of a circle 2 x Pi x r, therefore circumference of the Formula-e tire of .68m diameter = 2.136m

    1. DanC says:

      Circumference given diameter is C = Pi * d. He said diameter – not radius.

  3. speculawyer says:

    Because they can.

    ICE cars would do it if they could but they can’t do it efficiently. Why have a clumsy, expensive, prone-to-break transmission if you can avoid it. Entropy will get you eventually.

    1. Eric p says:

      Even if they could, not sure anybody would want to ride in an ICE car revving on the border of redline for hours on the highway. Noise level of electric motor is another big reason why 1 gear is possible.

    2. John says:

      Electric tow motors have been in production for 40 years.

      Everyone uses propane instead because electric motors cannot handle large loads. They’re always broken down.

    3. SJC says:

      Motors have torque starting at low RPM, thus they don’t NEED a multi speed gear box.

  4. WARREN says:

    Many people don’t realize the i8 has a 2 speed transaxle for the front electric motor. Furthermore, the 330e, 530e, X5e, and 740e, all go through the 8 speed transmission even when on all electric mode.

    1. CDAVIS says:

      @WARREN said: “…the 330e, 530e, X5e, and 740e, all go through the 8 speed transmission even when on all electric mode.”
      —–

      That’s Interesting.. I wonder why BMW decided to keep in their EVs a more traditional (ICEish) complicated & heavy transmission?

      1. CDAVIS says:

        … disregard my question. It makes sense considering those are hybrid EVs needing to accommodate ICE power at same time as electric motor power. I now remember that Chevy Volt dealt with this using a PG transmission.

        1. Pushmi-Pullyu says:

          Yeah, I expect those are all hybrid cars, and thus do need a gasmobile’s transmission to support the gas motor when it’s engaged in the drivetrain.

          This really underscores the simplicity of the pure EV, which will soon lead to EVs being lower cost than comparable gasmobiles.

        2. WARREN says:

          However, the i8 2 speed GKN for its front electric motor setup is physically independent of the rear gas motor.

  5. Don Zenga says:

    So how much is the cost savings with transmission in an EV compared to ICE.

    I believe the cost of a motor is $2,000 while that of an engine is $4,000

  6. SparkEV says:

    SparkEV gear ratio is 3.89:1. I think that’s the tallest gear (lowest ratio) among any EV. It’s such a unique beast.

    1. georgeS says:

      @Sparkev,

      That’s because SparkEV’s motor has very high torque. The other unique thing is that SparkEV used a planetary gear reduction set as opposed to straight cut gears.

      GM eliminated both in the BoltEV which now has low cost straight cut gears and not so tall a ratio.

      http://insideevs.com/chevy-bolt-200-mile-ev-battery-cooling-and-gearbox-details-bower/

      1. SparkEV says:

        Some Tesla motors make more torque than SparkEV, yet their gear ratio is much higher.

        By the way, SparkEV is still the highest torque motor outside of Tesla. Not quite as unique as gear ratio, but still impressive.

        1. ModernMarvelFan says:

          Spark EV has more torque but very low “max” RPM… So, it has to have low gearing ratio in order to reach hwy speed. =)

      2. Adam says:

        Those gears are not straight cut, but rather helical. No road car would ever employ straight cut gears as the noise would drive you bonkers.

        Helical gears are much quieter, but create centrifical forces that require stronger casings, bearings etc to overcome. Race cars use simpler straight cut gears because noise is a non-issue and gearboxes therefore are easier to make stronger.

  7. Bill Howland says:

    This guy is making a contradictory statement, in that the winner of the race had a gear box.

    Also, a gear box can ADD efficiency, not subtract from it if the added efficiency is greater than loss from the transmission.

    This is why all VOLTS (and ELRs) have CVTs.

    The reason is the loss at low speeds with only one speed CAN be high if the motor is not accelerated slowly. My 1 speed roadster, if driven HARD would only go around 50 miles per charge – no where close to its epa 244 mile rating.

    This race proves a 3 speed gearbox improves efficiency.

    The volt could use a bit higher gearing here than even it has, but things are no so bad if you accelerate slowly until around 25 mph as ‘coached’ by the Green Leaf.

    The BOLT ev gets around this issue by having such a relatively HUGE motor, that the losses at low speeds under normal conditions are not that great, due to the relatively large wiring in the stator and Inverters, nor is its ‘oversized’ reduction gearing greatly taxed – plus the fact that the BOLT ev was designed to be Cheaply contructed, and therefore the cheapest and simplest approach (consistant with efficiency and reliability) was taken.

    Having a huge motor satisfies the drag ev racers, but it helps with the lead foot crowd also.

    The relatively smaller Voltec stuff requires a transmission for peak performance and efficiency – as in the winning race car.

    1. QCO says:

      Yes… What the author failed to conclude is the optimal solution is a balance between cost, efficiency and requirement for a particular scenario.

      An e-racer maximising peak power can afford a couple of low end gears. A Volt needs wide range gearing to tie in the engine operating at peak efficiencies. A Bolt matches motor operation to a single speed reduction gear to minimise cost.

      Same trade offs apply to ICE vehicles as well. In the early days efficiency took a back seat to cost when GM used 2 speed Powerglide automatics with loose torque converters. Now we have 8 speed automatic transmissions to maximise fuel efficiency by operating engines in a very narrow peak efficiency rpm band.

      Drivetrain engineering 101.

      1. Bill Howland says:

        Its only incidentally there for the engine. Most of the time when in EV only mode, the car fully utilizes the variable drive ratios, by adjusting the percentage power and speed to the two electric motors.

        1. QCO says:

          Definately. If the engine wasn’t there they would not have put in that wide ranging dual motor planetary gear set capable of optimising engine speed to match its power curve.

  8. Dave says:

    This is just another reason that incumbent ICE manufacturers are going to have huge costs associated with closing down no longer required transmission plants as EV’s take off.

    Transmission assembly plants are expensive and labor intensive. Many will be closed as EV’s sales replace ICE autos

  9. Pushmi-Pullyu says:

    Rather ironic for Formula E to be involved in an explanation of why EVs typically have a fixed gear ratio, when Formula E race cars have (according to Mr. Google) 3-5 gear ratio transmissions!

    😀

  10. Nada says:

    Engineering Explained = Awsome!!

    FYI…
    He said the team car that won the championship had a single speed and the driver that won the chamionship had a three speed…
    Not contradictory…
    https://en.m.wikipedia.org/wiki/2016%E2%80%9317_Formula_E_season
    2016–17 Formula E season
    Drivers’ Champion: Lucas di Grassi drives an Audi…
    Teams’ Champion: Renault e.dams

  11. Reboot says:

    Thumbs up. Nice video.

  12. jim stack says:

    It’s because an electric motor has a much bigger RPM range. It can go 1 mph or 160 mpg with no problem . A gas engine would fall apart at high RPM.

    Also an AC Electric motor has just as much torque at 1 rpm as 16,000 rpm and is very efficient all the way from low to high RPM. I can also switch to REGENerative and make energy while slowing or stopping reducing brake wear and brake dust !

    1. Doggydogworld says:

      EV motors have much MORE torque at 0 rpm than at max RPM.

      1. Pushmi-Pullyu says:

        Here’s a graph of power and torque curves for the Tesla Model S, from a 2015 post. Looks like torque drops off above ~60 MPH. I guess the graph is for the (single motor) Model S 85, altho the page doesn’t specify:

        Obviously other EVs, and other versions of the Model S, will have different torque curves.

        1. qCO says:

          Those curves represent the protection constraints programmed in the controller rather than any raw motor characteristics.

          For example, torque is limited to protect the driveline components and motor winding currents. And power is limited to protect the battery and match cooling system capacity.

      2. QCO says:

        “…motors have much MORE torque at 0 rpm than at max RPM…”

        Naturally, since power = speed x torque, so if one goes down the other goes up.

        But in practice we use controllers to place many constraints on motor operation to protect both the motor and driveline components from a variety of failure and longevity issues, so it’s never quite as simple as it may seem in theory.

        Motors have to be protected from over current and heating associated with higher torque at low speeds, and protected from exploding rotors due to centrifugal force at high speeds.
        Motor bearings, gears and joints have to be protected from excessive wear due to high speed or torque.
        Batteries have to be protected from excessive current and power draws.
        Controllers have their own power/current/voltage operating constraints.
        Cooling system capabilities will limit both continuous and intermittent power.

        All of these pieces have to be cohesively engineered to meet the design objectives, which may or may not include tire shredding stoplight starts.

    2. Bill Howland says:

      “same torque at 1 rpm as at 16000…..”

      That is ONLY true if 16,000 is the motor’s base speed,- but 99 times out of 100 that is not so, as 16,000 is rather fast for an electric motor also.

      If youre regularly going faster than that, you need a gas turbine.

    3. QCO says:

      Electric motor speeds are limited by design, not principal.

      Maximum rpm is limited by allowable rotor centrifugal force and bearing capabilities, both of which are motor design parameters set by requirement (and cost).

  13. ModernMarvelFan says:

    One thing people forget is that Max torque at 0RPM isn’t max power at 0RPM.

    Gears help to keep the car in “max power” range. That is why formula E still uses gears so in certain speed of the cornering, the car are always within its “max power” band.

    Of course, for daily commute,you don’t need to stay in max power band from 10mph all the way to 90mph..

    1. QCO says:

      Yep. In fact, infinite torque at zero rpm is zero power. Which also implies that for constant power an infinitesimally small rpm would require an infinitely large current.

      For an electric motor, “max power” range in practice means keeping it below the maximum current limits at low speeds and below maximum rpm limits. Which is why the ev racer benefits from lower gearing in low speed corners since it keeps motor rpm up and current down, therefore allowing operation at maximum power.

      Of course tire adhesion is another issue altogether, which necessitates a control function to detect and limit wheel slip by modulating maximum power.

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